IL-12 is a cytokine that links innate and adaptive immunity. Its subunit p40 is induced in macrophages following IFN-γ/LPS stimulation. Here we studied the role for IFN consensus sequence binding protein (ICSBP), an IFN-γ/LPS-inducible transcription factor of the IFN regulatory factor (IRF) family in IL-12 p40 transcription. Macrophage-like cells established from ICSBP−/− mice did not induce IL-12 p40 transcripts, nor stimulated IL-12 p40 promoter activity after IFN-γ/LPS stimulation, although induction of other inducible genes was normal in these cells. Transfection of ICSBP led to a marked induction of both human and mouse IL-12 p40 promoter activities in ICSBP+/+ and ICSBP−/− cells, even in the absence of IFN-γ/LPS stimulation. Whereas IRF-1 alone was without effect, synergistic enhancement of promoter activity was observed following cotransfection of ICSBP and IRF-1. Deletion analysis of the human promoter indicated that the Ets site, known to be important for activation by IFN-γ/LPS, also plays a role in the ICSBP activation of IL-12 p40. A DNA affinity binding assay revealed that endogenous ICSBP is recruited to the Ets site through protein-protein interaction. Last, transfection of ISCBP alone led to induction of the endogenous IL-12 p40 mRNA in the absence of IFN-γ and LPS. Taken together, our results show that ICSBP induced by IFN-γ/LPS, acts as a principal activator of IL-12p40 transcription in macrophages.

Mice lacking the transcription factor interferon consensus sequence binding protein (ICSBP), a member of the interferon regulatory factor family of transcription proteins, were infected with the intracellular protozoan, Toxoplasma gondii. ICSBP-deficient mice exhibited unchecked parasite replication in vivo and rapidly succumbed within 14 d after inoculation with an avirulent Toxoplasma strain. In contrast, few intracellular parasites were observed in wild-type littermates and these animals survived for at least 60 d after infection. Analysis of cytokine synthesis in vitro and in vivo revealed a major deficiency in the expression of both interferon (IFN)-γ and interleukin (IL)-12 p40 in the T. gondii exposed ICSBP−/− animals. In related experiments, macrophages from uninfected ICSBP−/− mice were shown to display a selective impairment in the mRNA expression of IL-12 p40 but not IL-1α, IL-1β, IL-1Ra, IL-6, IL-10, or TNF-α in response to live parasites, parasite antigen, lipopolysaccharide, or Staphylococcus aureus. This selective defect in IL-12 p40 production was observed regardless of whether the macrophages had been primed with IFN-γ. We hypothesize that the impaired synthesis of IL-12 p40 in ICSBP−/− animals is the primary lesion responsible for the loss in resistance to T. gondii because IFN-γ–induced parasite killing was unimpaired in vitro and, more importantly, administration of exogenous IL-12 in vivo significantly prolonged survival of the infected mice. Together these findings implicate ICSBP as a major transcription factor which directly or indirectly regulates IL-12 p40 gene activation and, as a consequence, IFN-γ–dependent host resistance.

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IRF-8/ICSBP and IRF-1 are IRF family members whose expression is induced in response to IFN-Q Q in macrophages. IL-12 is a cytokine produced in macrophages that plays a critical role in host defense. IFN-Q Q and bacterial lipopolysaccharide (LPS) induce IL-12p40 transcription, which is necessary for the production of IL-12. We have previously shown that IL12p40 expression is impaired in ICSBP-de¢cient mice and that transfection of ICSBP together with IRF-1 can activate IL12p40 expression in mouse macrophage cells. To further study the role of ICSBP and IRF-1, we investigated murine IL-12p40 promoter activity in the macrophage cell line RAW 264.7. We show here that co-transfection of ICSBP and IRF-1 synergistically stimulates IL-12 promoter activity to a level comparable to that induced by IFN-Q Q/LPS. Mutation of the Ets or NFU UB site previously shown to be important for IL-12p40 transcription did not abolish the activation by ICSBP and IRF-1. However, mutation of the ISRE-like site found downstream from the NFU UB and C/EBP sites abrogated the activation by ICSBP and IRF-1. Together, these results indicate that ICSBP and IRF-1 cooperatively stimulate murine IL-12 transcription through a novel regulatory element in the murine promoter.